Thus, an actuator, commonly known as a pneumatic actuator, comprises an actuator piston disc that is displaceable in axial direction between a first position (inactive position) and a second position (active/extended position). The displacement is achieved by controlling a supply of pressure fluid, such as pressurized gas/air, that acts on and drives the actuator piston disc. The actuator piston disc acts in its turn directly or indirectly on the object that is to be displaced, for example an engine valve, for controlling its position.
In the application having an engine valve, when the actuator piston disc is in the inactive position the engine valve is in contact with its seat, and when the actuator piston disc is in the active position the engine valve is open, i.e. located at a distance from its seat.
In known actuators the inlet valve body is as a rule constituted by a slide valve body that is driven by an electro magnet. In for instance U.S. Pat. No. 8,973,541 it is disclosed that a first inlet valve and an outlet valve are connected to each other and constitute part of one and the same slide valve body, wherein the slide valve body is either driven directly by the electro member or the electro member controls the slide valve body indirectly via an unspecified and indefinite “pilot-slave” arrangement, which is not at all described in said document. However, it is plausible that such a “pilot-slave-arrangement” embrace that the electro member displaces a “pilot” that indirectly drive the slide valve body being a “slave” by the electro member/pilot controls the supply of a pressure fluid acting on and displaces the slide valve body, as is shown in for instance U.S. Pat. No. 3,727,595.
One drawback of the disclosed design having the first inlet valve and the second inlet valve combined in one and the same slide valve body, is that the slide valve body has great mass that counteract fast acceleration in connection with the actuator is given instruction regarding change of operational condition, from inactive to active. In other words the change over time from closed inlet channel to fully open inlet channel is unnecessarily long, and the initial movement of the slide valve body when the inlet channel is to be opened is the slowest which is the opposite to the wanted. Thereto, a so-called draught will occur when the seat valve body is in motion from the first position to the second position. Draught entail that the inlet is open at the same time as the outlet is still open, leading to the pressure fluid passing straight through the actuator without performing any useful work. The only way to prevent draught in the disclosed design, in which a common slide valve body is used, is to not open the inlet channel when the slide valve body starts to move until after the outlet channel has been closed, that entail unnecessary long displacement of the slide valve body which takes time and consumes unnecessary amounts of pressure fluid.
The greatest problem is that it always takes place a certain leakage of pressure fluid past the slide valve body which also entails increased consumption of pressure fluid performing no useful work. All consumption of pressure fluid is directly connected to the energy consumption of the actuator and thereby it is central to keep the consumption to a minimum.